In general, an internet service, such as electronic transaction, internet banking, and online game, requires a user to register such service and to be authorized. User authentication includes, for example, receiving user identification (ID) and password through a web browser of a user terminal, transmitting them to a server, and determining whether the received information matches information stored in the server. Further, a password is also required in a mobile information processing device or a computer to execute some specified functions. A password includes characters, numbers, symbols, or a combination thereof.